Ethane oxidation catalyst and process utilising the catalyst

ABSTRACT

A catalyst composition and its use for the selective oxidation of ethane to acetic acid and/or for the selective oxidation of ethylene to acetic acid which composition comprises in combination with oxygen the elements molybdenum, vanadium, niobium, gold in the absence of palladium according to the empirical formula: Mo a W b Au c V d Nb e Z f  wherein Z is one or more elements selected from the group consisting of B, Al, Ga, In, Ge, Sn, Pb, Sb, Cu, Pt, Ag, Fe and Re; a, b, c, d, e and f represent the gram atom ratios of the elements such that: 0&lt;a≦1; 0≦b&lt;1 and a+b=1; 10 −5 φc≦0.02; 0&lt;d≦2; 0&lt;e≦1; and 0.0001≦f≦0.05.

[0001] The present invention relates to a catalyst for the selectiveoxidation of ethane to acetic acid and/or for the selective oxidation ofethylene to acetic acid, and to a process for the production of aceticacid utilising the aforesaid catalyst.

[0002] Catalysts comprising molybdenum, vanadium and niobium incombination with oxygen for use in processes for the production ofacetic acid by the oxidation of ethane and ethylene are known in the artfrom, for example, U.S. Pat. No. 4,250,346, EP-A-1043064, WO 99/20592and DE 196 30 832.

[0003] U.S. Pat. No. 4,250,346 discloses the oxidative dehydrogenationof ethane to ethylene in a gas phase reaction at relatively high levelsof conversion, selectivity and productivity to ethylene at a temperatureof less than about 550° C. using as a catalyst a composition comprisingthe elements molybdenum, X and Y in the ratio Mo_(a)X_(b)Y_(c) wherein Xis Cr, Mn, Nb, Ta, Ti, V and/or W, and preferably Mn, Nb, V and/or W; Yis Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U, andpreferably Sb, Ce and/or U, a is 1, b is 0.05 to 1.0 and c is 0 to 2,and preferably 0.05 to 1.0, with the proviso that the total value of cfor Co, Ni and/or Fe is less than 0.5.

[0004] WO 99/20592 relates to a method of selectively producing aceticacid from ethane, ethylene or mixtures thereof and oxygen at hightemperature in the presence of a catalyst having the formulaMo_(a)Pd_(b)X_(c)Y_(d) wherein X represents one or several of Cr, Mn,Nb, Ta, Ti, V, Te and W; Y represents one or several of B, Al, Ga, In,Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg,Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl and U and a=1,b=0.0001 to 0.01, c=0.4 to 1 and d=0.005 to 1.

[0005] German patent application DE 196 30 832 A1 relates to a similarcatalyst composition in which a=1, b>0, c>0 and d=0 to 2. Preferably,a=1, b=0.0001 to 0.5, c=0.1 to 1.0 and d=0 to 1.0.

[0006] The catalysts of both WO 99/20592 and DE 19630832 require thepresence of palladium.

[0007] EP-A-1043064 discloses a catalyst composition for the oxidationof ethane to ethylene and/or acetic acid and/or for the oxidation ofethylene to acetic acid which comprises in combination with oxygen theelements molybdenum, vanadium, niobium and gold in the absence ofpalladium according to the empirical formula:

Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Y_(f)  (I)

[0008] wherein Y is one or more elements selected from the groupconsisting of: Cr, Mn, Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co,Rh, Ir, Cu, Ag, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P,Pb, Sb, Si, Sn, Tl, U, Re, Te, La and Pd; a, b, c, d, e and f representthe gram atom ratios of the elements such that: 0<a≦1; 0≦b<1 and a+b=1;10⁻⁵<c≦0.02; 0<d≦2; 0<e≦1; and 0≦f≦2.

[0009] There remains a need to develop a catalyst for the oxidation ofethane and/or ethylene to acetic acid and a process for the productionof acetic acid using said catalyst and wherein the catalyst enables ahigh selectivity to acetic acid to be achieved.

[0010] Surprisingly, it has now been found that it is possible by usinga catalyst that contains, in combination with oxygen, the elementsmolybdenum, vanadium, niobium and gold and one or more elements selectedfrom the group consisting of boron, aluminium, gallium, indium,germanium, tin, lead, antimony, copper, platinum, silver, iron andrhenium, in the absence of palladium, to oxidise ethane and/or ethyleneto acetic acid with a high selectivity to acetic acid. Furthermore, ithas been found possible using the catalysts of the present invention, toachieve a high selectivity to acetic acid with reduced, for example,little, if any, selectivity to ethylene.

[0011] Accordingly, the present invention provides a catalystcomposition for the selective oxidation of ethane to acetic acid and/orfor the selective oxidation of ethylene to acetic acid which compositioncomprises in combination with oxygen the elements molybdenum, vanadium,niobium, gold in the absence of palladium according to the empiricalformula:

Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Z_(f)  (I)

[0012] wherein Z is one or more elements selected from the groupconsisting of B, Al, Ga, In, Ge, Sn, Pb, Sb, Cu, Pt, Ag, Fe and Re;

[0013] a, b, c, d, e and f represent the gram atom ratios of theelements such that:

[0014] 0<a≦1; 0≦b<1 and a+b=1;

[0015] 10⁻⁵<c≦0.02;

[0016] 0<d≦2;

[0017] 0<e≦1; and

[0018] 0.0001≦f≦0.05

[0019] Catalysts embraced within the formula (I) include:-

[0020] Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Sn_(f)

[0021] Mo_(a.)Au_(c)V_(d)Nb_(e)Sn_(f)

[0022] Preferably, Z is Sn, Ag, Fe or Re, especially Sn.

[0023] Examples of suitable catalysts having the formula (I) include:-

[0024] Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Ag_(0.008)O_(y),

[0025] Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Fe_(0.0156)O_(y),

[0026] Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Re_(0.008)O_(y),

[0027] Mo_(1.000)V_(0.423)Nb_(0.115.)Au_(0.0008)Sn_(0.0008)O_(y), and

[0028] Mo_(1.000)V_(0.423)Nb_(0.117.)Au_(0.0008)Sn_(0.0156)O_(y) whereiny is a number which satisfies the valencies of the elements in thecomposition for oxygen.

[0029] Preferably 0.01<a≦1. Preferably, 0.1<d≦2. Preferably, 0.01<e≦0.5,for example, 0.05≦e≦0.15. Preferably, 0.0005≦f≦0.02.

[0030] An advantage of the catalyst compositions of the presentinvention is that they are highly selective in converting ethane and/orethylene to acetic acid. Typically, using the catalyst compositions ofthe present invention, a selectivity to acetic acid of at least 50 mol%, preferably at least 60 mol %, such as at least 70 mol %, may beachieved.

[0031] In particular, using the catalyst compositions of the presentinvention, a high selectivity to acetic acid may be achieved incombination with a low, if any, selectivity to ethylene.

[0032] Typically, using the catalyst compositions of the presentinvention, the selectivity to ethylene is less than 25 mol %,preferably, less than 10 mol %, such as less than 5 mol %.

[0033] Preferably, using the catalyst compositions of the presentinvention, the selectivity to acetic acid is at least 60 mol %, such asat least 70 mol % and the selectivity to ethylene is less than 15 mol %,such as less than 10 mol %.

[0034] As used herein, selectivity refers to a percentage that reflectsthe amount of desired acetic acid product produced as compared to thetotal carbon in the products formed:-

% selectivity=100*Moles of acetic acid produced/S

[0035] wherein S=the molar acid-equivalent sum (carbon basis) of allcarbon-containing products, excluding the alkane in the effluent

[0036] The catalyst compositions may be prepared by any of the methodsconventionally employed for the preparation of catalysts. Suitably thecatalyst may be prepared from a solution of soluble compounds and/orcomplexes and/or compounds of each of the metals. The solution ispreferably an aqueous system having a pH in the range from 1 to 12,preferably from 2 to 8, at a temperature of from 20° to 100° C.

[0037] Generally, a mixture of compounds containing the elements isprepared by dissolving sufficient quantities of soluble compounds anddispersing any insoluble compounds so as to provide a desired gram-atomratio of the elements in the catalyst composition. The catalystcomposition may then be prepared by removing the solvent from themixture. The catalyst may be calcined by heating to a temperature offrom 200 to 550° C., suitably in air or oxygen, for a period of from 1minute to 24 hours. Preferably, the air or oxygen is slowly flowing.

[0038] The catalyst may be used unsupported or supported. Suitablesupports include silica, alumina, zirconia, titania, silicon carbide andmixtures of two or more thereof.

[0039] Further details of a suitable method for preparing a catalystcomposition may be found in, for example, EP-A-0166438.

[0040] The catalyst may be used in the form of a fixed or a fluidisedbed.

[0041] In another embodiment the present invention provides a processfor the selective production of acetic acid from a gaseous mixturecomprising ethane and/or ethylene which process comprises contacting thegaseous mixture with a molecular oxygen-containing gas at elevatedtemperature in the presence of a catalyst composition as hereinbeforedescribed.

[0042] Ethane is selectively oxidised to acetic acid and/or ethylene isselectively oxidised to acetic acid. Preferably, ethane and optionallyethylene is oxidised to a mixture comprising acetic acid which may beused with or without the addition or removal of acetic acid for theproduction of vinyl acetate by reaction with a molecularoxygen-containing gas in an integrated process.

[0043] The feed gas comprises ethane and/or ethylene, preferably ethane.

[0044] Ethane and/or ethylene may be used in substantially pure form oradmixed with one or more of nitrogen, methane, carbon dioxide and waterin the form of steam, which may be present in major amounts, for examplegreater than 5 volume percent or one or more of hydrogen, carbonmonoxide, C₃/C₄ alkenes and alkenes, which may be present in minoramounts, for example less than 5 volume percent.

[0045] The molecular oxygen-containing gas may be air or a gas richer orpoorer in molecular oxygen than air, for example oxygen. A suitable gasmay be, for example, oxygen diluted with a suitable diluent, for examplenitrogen.

[0046] It is preferred to feed, in addition to ethane and/or ethyleneand the molecular oxygen-containing gas, water (steam) because this canimprove the selectivity to acetic acid.

[0047] The elevated temperature may suitably be in the range from 200 to500° C., preferably from 200 to 400° C.

[0048] The pressure may suitably be atmospheric or superatmospheric, forexample in the range from 1 to 50 bar, preferably from 1 to 30 bar.

[0049] The catalyst composition is preferably calcined before use in theprocess of the invention. Calcination may suitably be achieved byheating at a temperature suitably in the range from 250 to 500° C. inthe presence of an oxygen-containing gas, for example air.

[0050] Operating conditions and other information applicable to theperformance of the invention may be found in the aforesaid prior art,for example U.S. Pat. No. 4,250,346.

[0051] The process of the invention will now be further illustrated byreference to the following Examples.

CATALYST PREPARATION Preparation of Catalyst A (Comparative)

[0052] A solution ‘A’ was prepared by dissolving 22.935 g of ammoniummolybdate and 0.0357 g of ammonium gold chloride in 100 ml of distilledwater at 70° C. with stirring. A solution ‘B’ was prepared by dissolving6.434 g of ammonium vanadate in 150 ml of distilled water at 70° C. withstirring. A solution ‘C’ was prepared by dissolving 7.785 g of ammoniumniobium oxalate in 100 ml of distilled water at 70° C. with stirring.Each of the solutions A, B and C was left for 15 minutes to allowmaximum solubilisation of the components. Solution C was then added tosolution B rapidly with stirring at 70° C. The mixed solution B/C wasstirred for 15 minutes at 70° C. then added rapidly to solution A. Thefinal mixed solution A/B/C was left to stir at 70° C. for a further 15minutes, after which the solution was heated to boiling to facilitateevaporation of the water. Full evaporation of the reactant mixture wasachieved in 1.5 hours, resulting in a dry paste. The beaker with thedried paste was then transferred to an oven for further drying at 120°C. for 2 hours. After drying, the catalyst precursor was ground to afine powder and then sieved through a 0.2 mm mesh sieve. The resultingpowdered catalyst cake was then calcined in static air in an oven at400° C. for 4 hours. The nominal formula of the oxide catalyst obtainedwas:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.0008)O_(y)

[0053] This catalyst is not a catalyst according to the inventionbecause it contains no element from the group consisting of B, Al, Ga,In, Ge, Sn, Pb, Sb, Cu, Pt, Ag, Fe and Re.

Preparation of Catalyst B

[0054] A catalyst B was prepared as for catalyst A except that 0.0190 gof tin (II) chloride was additionally added to solution A. The nominalformula of the oxide catalyst obtained was:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.0008)Sn_(0.0008)O_(y)

Preparation of Catalyst C

[0055] A catalyst C was prepared as for catalyst A except that 0.3792 gof tin (II) chloride was added to solution A. The nominal formula of theoxide catalyst obtained was:

Mo_(1.000)V_(0.423)Nb_(0.117)Au_(0.0008)Sn_(0.0156)O_(y)

Preparation of catalyst D

[0056] A catalyst D was prepared as for catalyst A but with addition of0.0299 g of antimony (III) acetate (FW 298.88) to solution A. Thenominal formula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Sb_(0.008)O_(y)

Preparation of catalyst E

[0057] A catalyst E was prepared as for catalyst A but with addition of0.0200 g of copper (II) acetate (FW 199.65) to solution A. The nominalformula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Cu_(0.008)O_(y)

Preparation of catalyst F

[0058] A catalyst F was prepared as for catalyst A but with addition of0.0027 g of platinum acetate (FW 352.66) to solution A. The nominalformula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Pt_(0.0006)O_(y)

Preparation of catalyst G

[0059] A catalyst G was prepared as for catalyst A but with addition of0.0174 g of silver (I) acetate (FW 166.92) to solution A. The nominalformula of the oxide catalyst was-thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Ag_(0.008)O_(y)

Preparation of catalyst H

[0060] A catalyst H was prepared as for catalyst A but with addition of0.8080 g of ferric (III) nitrate (FW 404.00) to solution A. The nominalformula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Fe_(0.0156)O_(y)

Preparation of catalyst I

[0061] A catalyst I was prepared as for catalyst A but with addition of0.0268 g of ammonium rhenate (FW 268.24) to solution A. The nominalformula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Re_(0.008)O_(y)

Preparation of catalyst J

[0062] A catalyst J was prepared as for catalyst A but with addition of0.0256 g of gallium nitrate (FW 255.74) to solution A. The nominalformula of the oxide catalyst was thus:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Ga_(0.008)O_(y)

[0063] General Ethane Oxidation Reaction Method

[0064] Typically 5 ml of a powdered catalyst A to J was mixed with 15 mlof glass beads of diameter 0.4 mm to form a diluted catalyst bed of 20ml in volume. The diluted catalyst was then loaded into a fixed bedreactor made of Hastelloy of dimensions 12 mm internal diameter andlength 40 cm. The catalyst was maintained in position in the centre ofthe reactor using quartz wall plugs with inert packing material aboveand below the catalyst bed. The apparatus was then pressure-tested at 20bar with helium to check for leaks. The catalyst was then activated byheating to 220° C. at 5° C./min in helium at 16 bar for 1 hour, toensure full decomposition of catalyst precursors.

[0065] The required flows of ethane, ethylene, 20% oxygen in helium andwater were then introduced to the reactor, to ensure the required inletcomposition. This composition was 52% v/v ethane, 6.7% v/v oxygen, 10%v/v ethylene, 5% v/v water and balance helium. The total feed flow ratewas maintained at a level to ensure a feed GHSV of 2000-9000/h. Afterequilibrating for 60 minutes, gas samples were taken from the outletstream to a GC system (model Unicam 4400) to quantify ethane, ethylene,oxygen and helium.

[0066] The setpoint temperature of the reactor was increased to 293° C.,to achieve a similar reactor temperature of 299-301° C. for each ofcatalysts A-J, in order to facilitate direct comparison. Following afurther equilibration period of 60 minutes, liquid product collectionwas commenced and continued for a period of typically 18 hours. Duringthe run period, the effluent gas composition was measured using GCanalysis (ProGC, Unicam). Exit gas volume was measured over the runperiod by a water-gas meter. The liquid products were collected andweighed after the run period. Composition of the liquid products wasmeasured using gas chromatography analysis (Unicam 4400 and 4200 fittedwith TCD and FID detectors respectively).

[0067] From analysis of the feed and product flow rates and compositionsthe following parameters were calculated:

[0068] Conversions:

of ethane=(inlet mol ethane−outlet mol ethane)/inlet mol ethane*100

of oxygen=(inlet mol oxygen−outlet mol oxygen )/inlet mol oxygen*100

[0069] Selectivities:

to acetic acid (C-mol %)=(outlet mol acetic acid*2)/((outlet molethylene*2−inlet mol ethylene*2)+outlet mol CO+outlet mol CO₂+outlet molacetic acid*2)*100

to ethylene(C-mol %)=(outlet mol ethylene*2)/((outlet molethylene*2−inlet mol ethylene*2)+outlet mol CO+outlet mol CO₂+outlet molacetic acid*2)*100

to CO (C-mol %)=(outlet mol CO)/((outlet mol ethylene*2−inlet molethylene*2)+outlet mol CO+outlet mol CO₂+outlet mol acetic acid*2)*100

to CO₂(C-mol %)=(outlet mol CO₂)/((outlet mol ethylene*2−inlet molethylene*2)+outlet mol CO+outlet mol CO₂+outlet mol acetic acid*2)*100

to CO_(x)=selectivity to CO (C-mol %)+selectivity to CO₂(C-mol %) STY(space time yield)%=(g acetic acid)/kg catalyst bed/hour

[0070] Typically, mass balance and carbon balance for a reaction wasfound to be 100+/−5%.

Experiment A and Examples 1 to 9

[0071] Each catalyst A to J was employed in the general reaction methoddescribed above. The results are given in Tables 1. Each catalyst wasevaluated under standard conditions indicated in Table 1. TABLE 1 EthaneSTY Convn Sel. Acetic AcOH Experi- Cata- % C- acid Sel. C₂H₄ Sel CO_(x)g/kg- ment lyst mol % C-mol % C-mol % C-mol cat/h A A 7.8 47.0 34.4 18.5163 Exam- B 4.2 71.3 0.0 28.7 118 ple 1 Exam- C 3.8 70.8 0.0 29.2 105ple 2 Exam- D 6.3 56.9 24.7 18.5 133.6 ple 3 Exam- E 4.1 58.4 12.9 28.785.9 ple 4 Exam- F 5.1 59.9 12.6 27.4 112.7 ple 5 Exam- G 3.1 63.4 1.734.9 99.8 ple 6 Exam- H 4.7 68.1 5.6 26.4 119.3 ple 7 Exam- I 5.0 70.15.7 24.2 131.1 ple 8 Exam- J 5.8 53.9 27.5 18.7 115.8 ple 9

[0072] The results in Table 1 clearly demonstrate that compared to thecomparison catalyst, catalyst A, the catalysts of the present inventionachieve higher selectivities to acetic acid. Furthermore, a highselectivity to acetic acid is achieved in combination with reducedselectivity to ethylene.

1. A catalyst composition for the selective oxidation of ethane toacetic acid and/or for the selective oxidation of ethylene to aceticacid which composition comprises in combination with oxygen the elementsmolybdenum, vanadium, niobium, gold in the absence of palladiumaccording to the empirical formula:Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Z_(f)  (I)wherein Z is one or more elementsselected from the group consisting of B, Al, Ga, In, Ge, Sn, Pb, Sb, Cu,Pt, Ag, Fe and Re; a, b, c, d, e and f represent the gram atom ratios ofthe elements such that: 0<a≦1; 0≦b<1 and a+b=1; 10⁻⁵<c≦0.02; 0<d≦2;0<e≦1; and 0.0001≦f≦0.05
 2. A catalyst composition as claimed in claim 1wherein 0.01<a≦1,0.1<d≦2, 0.01<e≦0.5 and 0.0005≦f≦0.02.
 3. A catalystcomposition as claimed in claim 1 wherein Z is Sn, Ag, Fe or Re.
 4. Acatalyst composition as claimed in claim 3 wherein Z is Sn. 5 A catalystcomposition as claimed claim 4 wherein formula I is selected from thegroup consisting of Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Sn_(f) andMo_(a)Au_(c)V_(d)Nb_(e)Sn_(f).
 6. A catalyst composition as claimed inclaim 1 having the formula selected from the group consisting ofMo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Ag_(0.008)O_(y),Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)Fe_(0.0156)Q_(y),Mo_(1.000)V₀₄₂₃Nb_(0.115)Au_(0.008)Re_(0.008)O_(y),Mo_(1.00)V_(0.423)Nb_(0.115)Au_(0.0008)Sn_(0.0008)O_(y), andMo_(1.00)V_(0.423)Nb_(0.117)Au_(0.0008)Sn_(0.0156)O_(y), wherein y is anumber which satisfies the valencies of the elements in the compositionfor oxygen.
 7. A process for the production of acetic acid from agaseous mixture comprising ethane and/or ethylene which processcomprises contacting the gaseous mixture with a molecularoxygen-containing gas at elevated temperature in the presence of acatalyst composition as claimed in claim
 1. 8. process as claimed inclaim 7 in which ethane and optionally ethylene is oxidized to a mixturecomprising acetic acid.
 9. A process as claimed in claim 7 in which theelevated temperature is in the range from 200 to 500° C.
 10. A processas claimed in claim 7 in which the pressure is the range from 1 to 50bar.
 11. A process as claimed in claim 7 in which the selectivity of theoxidation reaction of ethane and/or ethylene to acetic acid is at least50 mol %.
 12. A process as claimed in claim 11 in which the selectivityof the oxidation reaction of ethane and/or ethylene to acetic acid is atleast 60 mol %.